1. Field of the Invention
The invention relates to a fuel cell system and a control method for a fuel cell system.
2. Description of Related Art
There has been practically used a fuel cell system that includes a fuel cell that is supplied with reaction gases (fuel gas and oxidation gas) to generate electric power. In the fuel cell system, for the purpose of ensuring stable power generation in the fuel cell, reaction gases slightly larger than respective gas amounts corresponding to a power (load request) required from a load device are generally supplied to the fuel cell. Here, the ratios of supplied gas amounts (actually, gas amounts supplied to the fuel cell) to respective reference gas amounts (gas amounts corresponding to the load request) may be referred to as “stoichiometric ratios”.
Currently, there is suggested a fuel cell system that includes means for calculating the stoichiometric ratios (cell stoichiometric ratios) of reaction gases for each of single cells that constitute a fuel cell and means for increasing the amount of supplied reaction gas when the corresponding cell stoichiometric ratio is lower than a predetermined value (for example, see Japanese Patent Application Publication No. 2007-184202 (JP 2007-184202 A)).
However, in the existing fuel cell system described in JP 2007-184202 A, it is possible to ensure desired stoichiometric ratios; however, it is not designed to suppress a power loss, so fuel, such as hydrogen gas, may be consumed unnecessarily. Particularly, currently, for the purpose of avoiding transitional fuel shortage (due to a delay in response of a circulation pump, or the like) during acceleration, a technique for keeping a high hydrogen concentration in a low load region is employed, so there is a problem that a fuel loss increases in the low load region.
In addition, in recent years, for the purpose of preventing drying of a fuel cell, there has been an attempt to increase the amount of water transferred from a cathode to an anode by reducing the thickness of an electrolyte membrane that constitutes the fuel cell. However, when the thickness of the electrolyte membrane is reduced in this way, there is a concern about a new problem that the amount of hydrogen gas that permeates from the anode to the cathode increases by that much and, as a result, hydrogen gas supplied for power generation is burned and consumed in the cathode without reacting with air. Thus, in order to employ such a thin electrolyte membrane, further technical innovation has been expected in order to suppress a fuel loss.